1. Field of the Invention
The present invention relates to a cutting tool made of a surface-coated cubic boron nitride-based ultrahigh pressure sintered material (hereinafter referred to as a surface-coated cBN-based sintered tool) comprising a cutting tool body made of a cubic boron nitride-based sintered material and a hard coating layer provided on the surface of the cutting tool body, which comprises a hard coating layer having excellent high-temperature hardness, high-temperature strength, heat resistance and edge notching resistance, and also can exhibit excellent edge notching resistance and maintain excellent surface finish over an extended period of time even in a high speed cutting operation of a hardened steel, such as alloy steel or hardened bearing steel.
This application claims priority on Japanese Patent Application No. 2005-370000 filed on Dec. 22, 2005 and Japanese Patent Application No. 2006-142108 filed on May 22, 2006, the disclosure of which is incorporated by reference herein.
2. Background Art
A surface-coated cBN-based sintered tool in general includes insert that is removably attached at the tip of a cutting tool for turning of workpieces made of various steels or cast iron, an insert type end mill that is removably attached with the indexable insert and is used in planning, grooving and shouldering of workpieces in a manner similar to that of the solid end mill.
One known constitution of surface-coated cBN-based sintered tools comprise a tool body made of various cubic boron nitride-based ultrahigh pressure sintered materials (hereinafter referred to as a cBN-based sintered material) and a surface-coating layer such as a Ti nitride (TiN) layer or a titanium aluminum nitride ([Ti, Al]N) layer formed by vapor deposition on the surface of the tool bodies. It is also known that these surface-coated cBN-based sintered tools are used for cutting workpieces made of various steels or cast irons (Japanese Unexamined Patent Application, First Publication No. Hei 7-300649).
It is also known that the above-described surface-coated cBN-based sintered tool can be manufactured by coating the surface of the tool body S with a layer of desirable composition, for example, TiN layer or [Ti, Al]N layer using an arc ion plating apparatus, as a variation of physical vapor deposition apparatus schematically exemplified by FIGS. 1A and 1B. For example, the surface coating may be formed by setting the tool body in the arc ion plating apparatus; generating arc discharge by supplying a current of 90 A between anodes 61,62 and cathodes (evaporation source) 31, 32 consisting of metallic Ti or a Ti—Al alloy of a predetermined composition, where the ambient temperature is maintained at, for example, 500° C. by a heater, while a nitrogen gas is introduced as a reaction gas into the apparatus so as to achieve a reaction atmosphere with a pressure of 2 Pa, and a bias voltage of −100 V, for example, is applied to the tool body (Japanese Unexamined Patent Application, First Publication No. Hei 8-119774).
There have recently been remarkable development and installation of FA (Factory Automatization) in cutting apparatus. On the other hand, there are still strong demands for labor saving, energy saving and cost reduction in cutting operations, resulting in a trend toward higher cutting speed. The conventional surface-coated cBN-based sintered tool, performs satisfactorily in machining of steels and cast irons under ordinary cutting conditions. However, when it is used in a high speed continuous cutting operation or a high speed interrupted cutting operation of a hardened steel, such as alloy steel or hardened bearing steel which has Vickers hardness (C scale) as high as 50 or more, abnormal damage (hereinafter referred to as edge notching) is caused at the boundary portion of the cutting edge by high heat generated at a cutting edge or drastically large mechanical load imposed intermittently and impulsively at the cutting edge. As a result, it becomes difficult to keep surface finish over an extended period of time, thus the cutting tool reaches the end of tool life in a relatively short period of time.